Catalytic carbon for oxidation of carbon monoxide in the presence of sulfur dioxide

ABSTRACT

A carbon supported catalyst used for carbon monoxide oxidation is chemically modified by treating the activated carbon support with an oxidizing agent and/or a hydrophobic compound prior to impregnation with the catalyst mixture. The thus treated catalytic carbon is capable of oxidizing carbon monoxide in an air stream containing sulfur dioxide over an extended period of time.

This application is a division of application Ser. No. 832,043, filedSept. 9, 1977, which is a continuation-in-part of application Ser. No.705,649, filed July 15, 1976, now abandoned, which iscontinuation-in-part of application Ser. No. 557,209, filed Mar. 12,1975, now abandoned.

BACKGROUND OF THE INVENTION

A number of compositions are known which catalyze the oxidation ofcarbon monoxide to carbon dioxide at ambient temperatures. Theeffectiveness of these catalyst compositions is, however, generallylimited to oxidation of carbon monoxide from relatively dry gas streamswhich are free from sulfur dioxide.

A commonly used catalyst composition for oxidation of carbon monoxide isa mixture of palladium and copper chlorides, PdCl₂ -CuCl₂, supported onactivated carbon or alumina, for example. This catalyst compositionpromotes carbon monoxide oxidation from moist air streams at roomtemperature with exceptional efficiency and for virtually an indefiniteperiod of time. However, if the air stream contains sulfur dioxide, evento the extent of a few parts per million by volume thereof, the catalystcomposition is quickly poisoned and will have only a very short usefullife. Moreover, exposure to sulfur dioxide causes irreversible damage tothe catalyst, as evidenced by the fact that degradation of the catalystactivity continues even after removal of the sulfur dioxide from thegaseous stream which the catalyst is being used to treat.

The precise reason for the rapid loss of catalytic oxidation activity ofa catalyst such as PdCl₂ -CuCl₂ supported on activated carbon in thepresence of small amounts of sulfur dioxide is not known. It has beenrecognized that the phenomenon of catalyst poisoning, that is, theability of trace amounts of some impurity to destroy the efficiency of acatalyst, is a consequence of the fact that catalytic activity is oftenconcentrated on a relatively small portion of the surface of a catalyst.These areas of catalytic activity, called active centers, are thought topreferentially adsorb the poisoning material. When the active area ofthe catalyst is small, it can be completely covered by small amounts ofthe poison, in this case, sulfur dioxide. On the other hand, activatedcarbon itself is known to catalyze the oxidation of sulfur dioxide tosulfur trioxide, and the latter may further react with moisture presenton the activated carbon to form sulfuric acid. Thus, the pores of theactivated carbon may become occluded by adsorption of either or both ofsulfur trioxide and sulfuric acid. Such occlusion of the activatedcarbon pores may result in covering of the catalyst compositionimpregnated thereon. Also, the catalyst composition may be subject toattack by the sulfur trioxide or the sulfuric acid which has beenformed. On the other hand, it has been found that when sulfuric acid isadded onto the catalyst directly, that the catalytic efficiency of theactivated carbon supported catalyst is not affected.

The present invention provides a method for overcoming this problem ofcatalyst inefficiency or poisoning produced by sulfur dioxide, andcomprises a method of treating the activated carbon support for thecatalyst so as to greatly improve the performance of the catalyst inoxidation of carbon monoxide, in the presence of sulfur dioxide.Particularly, the present invention provides a method for oxidativemodification of the activated carbon support, and impregnation thereofwith an inert hydrophobic compound, as well as the thus treated supporttogether with an oxidation catalyst impregnated thereon.

It is known in the art that an activated carbon supported Cd²⁺ catalystemployed in acetaldehyde synthesis can be pretreated with nitric acid togive improved performance. See Siedlewski et al., Chem. Stosow. (1973)19(2), 221-8 (Chemical Abstracts, Vol. 80, 412412). However, Cd is not acatalyst for carbon monoxide oxidation as used in the present invention.

SUMMARY OF THE INVENTION

The present invention relates to separate and combined steps ofoxidative modification whereby oxygen is added to the surface of anactivted carbon support and the reducing properties of the activatedcarbon support are substantially reduced, and treatment of an activatedcarbon support by impregnating the support with an inert hydrophobiccompound, said activated carbon support to form the support for acatalyst composition, particularly PdCl₂ -CuCl₂, with which the modifiedand treated activated carbon is to be impregnated. The activated carbonbase or support may be any commercially available activated carbon forvapor phase application use and granular activated carbons are employed.Particularly preferred, is type BPL granular activated carbonmanufactured by Pittsburgh Activated Carbon Division of CalgonCorporation. The size of the granular activated carbon should be from0.5 to 4.0 mm. diameter for the granules, and these are preferably from0.8 to 1.2 mm. in diameter. The preferred size corresponds to a meshsize of 12×40 U.S. Sieve Series.

The present invention relates particularly to oxidative modification ofthe activated carbon followed by impregnation of the activated carbonwith an inert hydrophobic compound. The thus treated activated carbonsupport is a part of this invention, as is the treated activated carbonsupport impregnated with a conventional catalyst composition used foroxidation of carbon monoxide.

The present invention also relates to the use of an activated carbonsupported catalyst prepared in accordance with the methods describedabove, to oxidize carbon monoxide in the presence of sulfur dioxide foran extended period of time. It has been found that in carrying out thiscatalytic oxidation of the carbon monoxide, that the presence of somemoisture is required, at least to the extent of about 0.5% to about 2.0%by volume of the inlet gas stream being treated. The amount orconcentration of sulfur dioxide present is essentially unimportant. Theextended life of the activated carbon support catalyst of the presentinvention, as compared to that for the same catalyst on an untreatedactivated carbon support, will remain proportionately essentially thesame, regardless of the sulfur dioxide concentration or total amount ofsulfur dioxide experienced.

Conventional catalyst compositions which may be supported on activatedcarbon and used for oxidation of carbon monoxide, and are thus useful inthe present invention, include the base metals Cu, Cr, Ni, Fe, and Mn,usually as the oxide; and the noble metals Au, Ag, Pt, Pd, and Rh, asthe element or an ill-defined oxide. The chloride salts of suchconventional metal catalysts for oxidation of carbon monoxide are rarelyemployed, with the notable exception of PdCl₂ -CuCl₂. Other salts ofthese metals can be employed, and various combinations of these metalsand metal salts are useful. For example, Cu-Cr and Cu-Cr-Ag combinationsare effective catalysts. The oxide, carbonate, hydroxide and nitrateforms of these metals may be employed. However, the specific metals andmetal salts enumerated above are merely illustrative since generally anyoxidative catalyst will function to a greater or lesser extent in thepresent invention. A number of factors will affect the performance ofsuch catalysts, including the presence of moisture and any of severalrecognized catalyst poisons. Such factors will tend to govern the choiceof catalyst. An especially preferred catalyst for use in the presentinvention is PdCl₂ -CuCl₂. The amount of catalyst loading of theactivated carbon support is not critical and may be in amounts overknown operative ranges.

The oxidative modification of the activated carbon support comprisestreating the carbon with a fluid oxidizing agent at ambient or elevatedtemperatures for a period of time sufficient to add oxygen to thesurface of the activated carbon, that is, sufficient to producechemisorption of the oxygen by the activated carbon support in an amountof from about 1% to about 5% by weight of the activated carbon. Theimprovement produced by the oxidative modification of the activatedcarbon support is not considered to be a threshold effect, and thus theimprovement is produced in response to the addition of very smallamounts of oxygen, even though this improvement may not be measureable.Generally, improvement increases proportionately to oxygen added.However, it is preferred that at least from about 1% to about 2% byweight of oxygen be added to the activated carbon support. The oxidativemodification of the present invention is readily distinguishable fromthe "oxidation" of a char which is carried out during a conventionalactivation process to produce an activated carbon. In such a process theoxidizing action of the activating gases selectively erodes the surfaceof the char so as to increase the surface area, develop greaterporosity, and leave the remaining atoms arranged in configurations thathave specific affinities. By contrast, the step of oxidativemodification of the present invention adds oxygen to the activatedcarbon surface, thereby decreasing the reducing properties of theactivated carbon support.

The desired amount of oxygen addition is determined readily in anapproximate way by the failure of the oxidized activated carbon toelectroplate gold, palladium, or silver from their corresponding aqueoussolutions. A reduction in the iodine number of the oxidized activatedcarbon of from about 10% to about 20%, as compared to the originalunoxidized activated carbon, may also be used as an approximate measureof the desired amount of oxygen addition to the activated carbon. Bothof these measurements are based on the elimination of the reducingproperties of the activated carbon resulting directly from the additionthereto of the oxygen. Of course, the amount of oxygen added may bedetermined quantitatively, although with somewhat more difficulty.

The treatment with an oxidizing agent may also modify to some extent theoriginal pore structure as well as the surface properties of theactivated carbon, and thus, it is theorized, may considerably diminishthe catalytic action of carbon for sulfur dioxide oxidation. However, itis also postulated that the improved performance of the oxidizedactivated carbon is due to one or a combination of the following: (a)improved dispersion of the Pd catalyst following the oxidativemodification; (b) alteration of the mechanism of carbon monoxideoxidation as a result of modification in the pore size, pore space andsurface properties of the oxidized activated carbon; (c) development ofactive centers in the activated carbon for carbon monoxide oxidation;(d) decrease in retention of water vapor inside the pore space of theactivated carbon supported catalyst, thus enabling the active centers ofthe catalyst to remain exposed for carbon monoxide oxidation; (e) areduction in the competition for active oxygen by any sulfur dioxidepresent; and (f) destruction of sites for sulfur dioxide oxidation,thereby reducing occlusion of the active sites of the catalyst forcarbon monoxide oxidation, by sulfur trioxide or sulfuric acid.

The oxidizing fluid employed in the oxidative modification treatment ofthe activated carbon may be liquid, or a gas or vapor, and is selectedfrom a number of known oxidizing compositions. Thus, for example, theoxidizing agent may be an aqueous solution of one or more membersselected from the group consisting of ammonium salts, borates, calciumoxide, chlorates and perchlorates, cyanides, ferric and ferrouscompounds, manganese dioxide, nickel salts, permanganate, persulfates,and oxyacids, for example, hypochlorous acid and nitric acid. Theoxidizing agent may be oxygen, either substantially pure oxygen, orsimply air or oxygen-enriched air. Where a so-called chemical oxidizingagent is employed, the oxidation treatment can be carried out at roomtemperature. Where oxygen, and particularly air is employed as theoxidizing agent, it will usually be necessary to carry out the oxidationprocedure at an elevated temperature of from about 100° to about 600°C., preferably from about 200° to about 300° C., and it will benecessary to employ the oxygen or air as a flowing stream.

Where the oxidizing agent is an oxyacid, for example, hypochlorous acidor nitric acid, it should be employed in a concentration of from 1 to 15weight %, preferably from 5 to 10 weight %, and in an amount, expressedas a weight ratio of acid to activated carbon, of from 1:1 to 5:1,preferably 2:1 to 3:1. The treatment with hypochlorous acid or nitricacid should be carried out from 1 to 6 hours, preferably from 2 to 3hours, and at a temperature of 25° to 120° C., preferably from 80° to100° C. As indicated above, the size of the granules of activated carbonshould be from 0.5 to 4.0 mm. in diameter, preferably, from 0.8 to 1.2mm. By following the various treatment conditions noted above, it ispossible to oxidatively modify the activated carbon support inaccordance with the present invention, while avoiding problems oferosion or actual destruction of the activated carbon support. Thus,weight loss in the activated carbon support does not occur.

Oxidative modification will be most readily accomplished on a commercialscale by heating in air. For such a process the parameters oftemperature, air flow rate, oxygen content, retention time, and soforth, are not critical and may be readily determined by routineexperimentation with a view toward the desired end result of oxygenaddition to the activated carbon. For oxidative modification carried outin the laboratory, nitric acid has been found to be an effective andpractical oxidizing agent. As indicated above, the oxidativemodification of the activated carbon support is generally consideredcomplete if a granule of the thus treated carbon will not electroplateeither gold, palladium, or silver from their respective aqueoussolutions. The completion of the oxidative modification is alsoapproximately determined by a 10 to 20 percent reduction in the iodinenumber of the thus treated carbon compared to the iodine number of theoriginal untreated activated carbon material.

The oxidative modification treatment of the activated carbon supportdescribed above can effectively increase the life of an oxidationcatalyst impregnated thereon to the extent of as much as a five-foldincrease or more over that of the same catalyst impregnated on anuntreated activated carbon support. It has also been found that the lifeof an oxidation catalyst impregnated on an activated carbon can beincreased to a similar extent by impregnating the activated carbonsupport with an inert hydrophobic compound. It is theorized that thesematerials increase catalyst life by rendering the pore surfaces of theactivated carbon support repellant to water, thus reducing theaccumulation of moisture in the pores. While carbon itself ishydrophobic to the extent that it is wet more readily by organicsolvents than by water, all carbons are not completely hydrophobic, andit has been recognized that some areas of the carbon surface havepartially hydrophilic characteristics.

Hydrophobic compounds useful in the present invention are generallythose which do not adsorb or absorb water. All such compounds areincluded broadly, but preference will limit the choice of hydrophobiccompounds to those which are readily applied to the activated carbonsupport, which are inert in the sense that they do not possess anydetrimental properties with respect to the catalytic carbon monoxideoxidation process described herein, and which are not easily desorbedduring the said oxidation process. Examples of such inert hydrophobiccompounds are aliphatic hydrocarbons of at least 14 carbon atoms,petroleum oils such as mineral oil, and aromatic hydrocarbons of atleast 10 carbon atoms. The aromatic hydrocarbons which may be employedinclude, for examle, naphthalene, anthracene, and phenanthrene and theirderivatives. It has been found, however, that increased efficiency isobtained with aromatic hydrocarbons having larger numbers of benzenerings. Thus, anthracene is preferred to naphthalene. Polymeric materialsmay also be employed, for examle, polytetrafluoroethylene and variouspolysiloxanes. The inert hydrophobic compound is impregnated into theactivated carbon support in an amount of from about 1% to about 20% byweight of the activated carbon, preferably in an amount of from about 5%to about 15% by weight. However, the improvement produced by the inerthydrophobic compound impregnation of the activated carbon support is notconsidered to be a threshold effect, and consequently the saidimprovement commences with the impregnation of even very small amountsof inert hydrophobic compound, although such improvement is notmeasureable.

The inert hydrophobic material may be impregnated into the activatedcarbon support either before or subsequent to addition of the catalystcomposition. The inert hydrophobic compound may be impregnated into theactivated carbon support in a number of ways, depending largely upon thechemical nature of the compound. The activated carbon may simply beimmersed in a mineral oil impregnant, for example. An aromatichydrocarbon such as naphthalene may be impregnated readily by vaporphase adsorption.

The catalyst composition, particularly PdCl₂ -CuCl₂, is readilyimpregnated into the activated carbon support simply by preparing ahydrochloric or other acid solution of the catalyst composition,admixing the same with the activated carbon for a sufficient time topermit adsorption of the desired amount of catalyst, and then drying theactivated carbon support. Where the hydrophobic compound is an aromatichydrocarbon such as naphthalene, it can then be impregnated into thecatalyst-impregnated activated carbon support by vapor phase adsorption.The aromatic hydrocarbon in solid crystalline form is allowed to sublimeto a vapor which is contacted with the activated carbon support in aclosed atmosphere. Vapors of the aromatic hydrocarbon penetrate theactivated carbon, where they are adsorbed and held.

Where the oxidative modification step and the inert hydrophobiccomposition impregnation treatment step are employed together, theoverall method for preparing activated carbon supported catalysts foroxidation of carbon monoxide in accordance with the present inventionwill thus follow two alternative routes, or series of step. The firststep for both routes will be oxidative modification of the activatedcarbon support. The second step will be impregnation with an inerthydrophobic compound by one route, or, alternatively impregnation withthe catalyst composition in the other route. The third step will be,respectively, impregnation with the catalyst composition in the firstroute, and impregnation with an inert hydrophobic compound in the secondroute.

The activated carbon supported catalysts of the present invention,produced by oxidative modification and inert hydrophobic compositionimpregnation treatment of the activated carbon support, have importantadvantages over catalysts for carbon monoxide oxidation supported onsuch materials as alumina, Al₂ O₃. Chief among these advantages is thefact that the alumina supported catalyst quickly becomes completely andirreversibly poisoned by sulfur dioxide. By contrast, the pretreatedactivated carbon supported catalysts of the present invention possessgreatly improved efficiency resulting from extended catalyst life in thepresence of sulfur dioxide.

The single FIGURE illustrates the improved efficiency of the pretreatedactivated carbon supported catalysts of the present invention over theperformance of an untreated activated carbon supported catalyst or analumina supported catalyst. The plots depicted essentially reflect thedata set out in Tables I and II hereinafter.

The following examples will serve to better illustrate the methods andcompositions of the present invention.

EXAMPLE 1

The activated carbon support employed throughout these procedures wasBPL type granular activated carbon from Pittsburgh Activated Carbon Co.,and was uniformly of size range 12×30 U.S. Sieve Series. Oxidativemodification of the activated carbon support was accomplished byrefluxing one volume of the BPL carbon with two volumes of 10%HNO₃solution at 110° C. for about four hours, followed by washing withdistilled water and drying at 150° C.

Untreated BPL carbon and BPL carbon oxidized as described above wereboth catalyst impregnated by adding to a known weight of dry untreatedand dry oxidized BPL carbon, a nearly equal weight of an HCl solution ofPdCl₂ -CuCl₂ sufficient to give a catalyst loading of the activatedcarbon support of 0.8% by weight of Pd and 10.0% by weight of CuCl₂.

Samples of both the untreated and the oxidized BPL carbons were thentreated with naphthalene and anthracene. For naphthalene impregnation, 2g. of naphthalene crystals were mixed together with 20 grams of eachcarbon sample and left in a closed container at room temperature untilall of the naphthalene had vaporized and the vapor adsorbed by theactivated carbon. A sample of the oxidized BPL carbon was impregnatedwith anthracene by mixing together 20 grams of the oxidized carbonsample and 2 grams of anthracene crystals, and leaving the mixture in aloosely closed container for one hour held at 125° C. The anthracene wasobserved to completely vaporize and be adsorbed by the carbon sample.The total impregnation of naphthalene and anthracene was thus 10% byweight.

The various samples prepared as described above were placed in 16 mm.internal diameter glass tubes to give a bed depth of 6.2 cm. Samplevolume was 11 ml. The samples were then exposed to a gas stream of airat ambient (80° F.) inlet temperature and 100% inlet relative humidity.The moisture content was achieved by bubbling the gas stream through aset of two water scrubbers. Carbon monoxide content of 0.5% by volume ofthe gas stream was achieved by mixing into the air stream a mixture of10% by volume of CO in N₂. A sulfur dioxide content of 200 ppm by volumein the gas stream was achieved by introducing into the gas stream amixture of 2400 ppm by volume of SO₂ in N₂. The flow rate of eachparticular final gas mixture was 550 ml/min. and each gas mixture waspassed downflow through the particular carbon sample in the glass tube.The glass tube sample reactor was wrapped with fiberglass insulation inorder to keep the heat of reaction inside the reactor. the sample runswere made at a space velocity of 3,000 bed volumes per hour (bvh-⁻¹).The results of the procedures performed as described above areillustrated in Table I below. The

                                      TABLE I                                     __________________________________________________________________________    WEIGHT PICKUP ON UNTREATED AND PRETREATED ACTIVATED                           CARBON SUPPORTED ON OXIDATION CATALYST                                                      Flow SO.sub.2                                                                           Time                                                                              Catalyst                                                                           % Wt.                                                                             Percent                                  Catalyst Type bvh.sup.-1                                                                         Present                                                                            Hours                                                                             Wt. (g)                                                                            Pickup                                                                            Removal                                  __________________________________________________________________________    0.8% Pd-10%                                                                   CuCl.sub.2 on activated                                                       carbon BPL     3,000                                                                             No   0    5.6 --  98.0                                                    3,000                                                                             No   20   6.1 8.95                                                                              97.0                                                    3,000                                                                             No   310  6.1 8.95                                                                              98.0                                     0.8% Pd-10%                                                                   CuCl.sub.2 on activated                                                       carbon BPL     3,000                                                                             Yes  0    5.9 --  --                                                      3,000                                                                             Yes  4   --   --  90                                                      3,000                                                                             Yes  49   8.7 50  28                                       0.8% Pd-10% CuCl.sub.2                                                        on HNO.sub.3 Oxidized                                                         BPL activated carbon                                                                         3,000                                                                             Yes  0    5.8 --  --                                                      3,000                                                                             Yes  40   7.4 27.6                                                                              90.7                                                    6,000                                                                             Yes  40.5                                                                              --   --  74.8                                                    3,000                                                                             Yes  42.0                                                                              --   --  87.0                                                    3,000                                                                             Yes  91  --   --  82.2                                                    3,000                                                                             Yes  110 10.3 83.5                                                                              10.0                                     0.8% Pd-10% CuCl.sub. 2 -10%                                                  Naphthalene on BPL                                                            activated carbon                                                                             3,000                                                                             Yes  0   6.6  --  --                                                      3,000                                                                             Yes  45  7.15  8.5                                                                              --                                                      3,000                                                                             Yes  75  7.95 20.0                                                                              91                                                      3,000                                                                             Yes  101 8.80 33.0                                                                              79                                                      3,000                                                                             Yes  115 9.30 40.9                                                                              61                                       0.8% Pd-10% CuCl.sub.2 on HNO.sub.3                                           Oxidized BPL activated                                                        carbon Contg. 10%                                                             Naphthalene    3,000                                                                             Yes  0   6.6  --  --                                                      3,000                                                                             Yes  4   --   --  92.2                                                    3,000                                                                             Yes  44  7.0   6.0                                                                              97                                                      3,000                                                                             Yes  75  7.4  12.0                                                                              96                                                      3,000                                                                             Yes  105 7.65 15.9                                                                              91                                                      3,000                                                                             Yes  153 8.85 34.0                                                                              75                                       0.8% Pd-10% CuCl.sub.2 on                                                     HNO.sub.3 Oxidized BPL                                                        activated carbon                                                              Contg. 10% Anthracene                                                                        3,000                                                                             Yes  0   6.65 --  --                                                      3,000                                                                             Yes  68  --   --  96.0                                                    3,000                                                                             Yes  93  7.5  12.8                                                                              96.0                                                    3,000                                                                             Yes  120 7.75 16.5                                                                              96.0                                                    3,000                                                                             Yes  144 8.30 25.0                                                                              85.0/94.2                                               3,000                                                                             Yes  165 8.65  30.08                                                                            87.0                                     SO.sub.2 removed from                                                         the stream     3,000                                                                             No   232 --   --  87.8                                                    3,000                                                                             No   300 9.70 45.9                                                                              60.6                                     __________________________________________________________________________

A number of observations and conclusions have been made based on thedata in Table I above. The increase in weight pickup, which consistsmostly of moisture, adversely affects the conversion (CO to CO₂)efficiency. It can also be seen that the rate of moisture buildup isstrongly influenced by the presence of SO₂ in the inlet stream. WhereSO₂ is absent from the inlet stream, the catalyst-impregnated BPLactivated carbon appears to have a virtually indefinite life since itdid not pick up more than 9% moisture even after 310 hours on stream, atwhich time it continued to remove more than 95% of the inlet CO. It canbe seen that the rate of moisture pickup decreases if the catalyst issupported on either an oxidized BPL activated carbon or a BPL activatedcarbon impregnated with naphthalene or anthracene. The naphthaleneimpregnation of the catalyst permits it to perform in the presence ofSO₂ approximately as well as the sample of catalyst supported on theoxidized BPL activated carbon. However, where the aromatic hydrocarbonimpregnation is combined with the oxidative modification of theactivated carbon support in forming the catalyst, it can be seen thatthe performance of the catalyst in the presence of SO₂ is increased inefficiency and longevity to a greater extent than the increase derivedfrom either individual treatment alone. The combined treatment of theactivated carbon catalyst support thus yields a synergistic improvementin the performance of the catalyst in the presence of SO₂. It can alsobe seen that the performance of the catalyst, as measured by the life ofthe catalyst and its level of CO oxidation, increases proportionatelywith the increasing number of benzene rings in the impregnant.Anthracene is thus seen to give better results than naphthalene. Thisresult can also be correlated with the decrease in rate of moisturebuildup.

EXAMPLE 2

In order to illustrate the improved performance of the pre-treatedactivated carbon supported catalysts of the present invention over theperformance of alumina supported catalysts for carbon monoxideoxidation, the procedures of Example 1 above were repeated using such analumina supported catalyst. This catalyst was 0.8% Pd (as chloride) and10% CuCl₂ by weight on alumina, Al₂ O₃, and was prepared in accordancewith the disclosure of Canadian Pat. No. 859,124. The results of theprocedures performed are illustrated in Table II below.

                  TABLE II                                                        ______________________________________                                        CO REMOVAL ON ALUMINA                                                         SUPPORTED CO OXIDATION CATALYST                                                          SO.sub.2      CO Levels, %-Vol.                                    Catalyst Flow    Level   Time       Out- %                                    Type     bvh.sup.-1                                                                            (ppm)   Hours Inlet                                                                              let  Removal                              ______________________________________                                                 3,000   None    5     0.55 None 100                                  0.8% Pd-10%                                                                            3,000   235     1.5   0.55 None 100                                  CuCl.sub.2 on                                                                          3,000   250     19.0  0.50 0.015                                                                              99                                   Al.sub.2 O.sub.3                                                                       3,000   200     20.0  0.50 0.075                                                                              98.5                                          3,000   150     38.0  0.50 0.458                                                                              8.3                                  ______________________________________                                    

Despite the importance of moisture buildup adverted to above, thisphenomenon is clearly not the sole reason for reduced catalystperformance in the presence of SO₂, as the following Example clearlyestablishes.

EXAMPLE 3

The partially exhausted catalyst samples of Example 1 above weresubjected to attempted regeneration by means of drying or calcining athigh temperatures. The regeneration conditions and results are set outin Table III below.

                                      TABLE III                                   __________________________________________________________________________    EFFECT OF REGENERATION ON CO OXIDATION                                                    Regen.        % Removal                                                       Temp.                                                                              Time                                                                             Moisture                                                                            Before  After                                       Catalyst Type                                                                             ° C.                                                                        Hrs.                                                                             Remaining                                                                           Regeneration*                                                                         Regeneration                                __________________________________________________________________________    0.8% Pd-10% CuCl.sub.2                                                        on oxidized BPL                                                               activated carbon                                                                          125  2  24.17 None    None                                           "        300  1  None  None    None                                        0.8% Pd-10% CuCl.sub.2 on                                                     BPL activated carbon                                                          10% Naphthalene                                                                           125  2  12.1  61.0    27.0                                        0.8% Pd-10% CuCl.sub.2 on                                                     oxidized activated                                                            carbon + 10%                                                                  Anthracene  Ambient                                                                            2  23.0  60.6    65.3                                        __________________________________________________________________________     *Removal performance of SO.sub.2 poisoned catalysts of Example 1.        

The data from Table III above also shows that drying accomplished byblowing with dry air at ambient temperatures did not worsen the rate ofdegradation, as was the case with attempted regeneration by drying orcalcining at high temperatures.

EXAMPLE 4

An experiment was carried out to determine the effects of oxidativemodification by nitric acid treatment on the activated carbon substratewith respect to weight loss of carbon. The nitric acid employed was 70weight % concentrated acid which was diluted with deionized water togive the concentrations for the experiment. Type BPL activated carbonfrom Calgon Corporation was employed in various mesh sizes, and wascharged to a flask to which various amounts and concentrations of nitricacid were then added. The activated carbon and acid were then stirredwith a magnetic stirrer for 3 hours. Where heating was employed, it wascarried out by use of a hotplate. After heating, or stirring at roomtemperature, the acid was decanted from the activated carbon, afterwhich the carbon samples were washed with one bed volume of deionizedwater by stirring for 10 minutes. The water was decanted and the washingprocedure repeated. The carbon samples were then dried overnight at 110°C. in an oven, after which they were weighed. The experimentalconditions and the results of the experiment are illustrated in thefollowing table of values:

                  TABLE IV                                                        ______________________________________                                        EFFECT OF ACID TREATMENT ON ACTIVATED                                         CARBON SUBSTRATE WEIGHT LOSS                                                                        Ac-                                                          Mesh             id.                                                          Size     Initial Car-       Con-                                              (U.S.    Carbon  bon  Acid  tact       %                                 Exp. Sieve    Weight  (wt.:                                                                              Conc. Time Temp. Wt.                               No.  Series)  (grams) wt.) (wt.%)                                                                              (hrs)                                                                              (°C.)                                                                        Change                            ______________________________________                                        1    100×140                                                                          21.94   10:1 60    3    80-100                                                                              -1.4                              2    100×140                                                                          21.45   10:1 20    3    25    +2.5                              3    12×30                                                                            25.0     2:1 10    3    80-100                                                                              +2.4                              ______________________________________                                    

What we claim is:
 1. A method of oxidizing carbon monoxide to carbondioxide in the presence of sulfur dioxide comprising contacting thecarbon monoxide with an activated carbon supported catalyst, produced byan improved method comprising the step of impregnating an activatedcarbon support with a catalyst composition comprising one or moremembers selected from the group consisting of Cu, Cr, Ni, Fe, Mn, Au,Ag, Pt, Pd, Rh and oxides, chloride, hydroxide, carbonate and nitratesalts thereof,the improvement comprising using an activated carbonoxidatively modified by heating said activated carbon support with afluid oxidizing agent prior to impregnating thereof with said catalyst,whereby oxygen is added to the surface of said activated carbon supportin an amount of from about 1% to about 5% by weight of said activatedcarbon support and the reducing properties of said activated carbonsupport are substantially reduced, provided that, where the oxidizingagent is an oxyacid, that the acid is employed in a concentration offrom 1 to 15 weight %, and in an amount, expressed as a weight ratio ofacid to activated carbon, of from 1:1 to 5:1, whereby weight loss fromthe activated carbon as a result of acid treatment does not occur, and,second, impregnating said activated carbon support with an inerthydrophobic compound in an amount of from about 1% to about 20% byweight of said activated carbon, before the step of impregnating saidactivated carbon support with said catalyst composition, saidimprovement resulting in an activated carbon supported catalyst havingimproved efficiency in the presence of sulfur dioxide.
 2. The method ofclaim 1 wherein in the method of producing the activated carbonsupported catalyst the hydrophobic compound is one or more membersselected from the group consisting of aliphatic hydrocarbons of at leastfourteen carbon atoms and aromatic hydrocarbons of at least ten carbonatoms.
 3. A method of oxidizing carbon monoxide to carbon dioxide in thepresence of sulfur dioxide comprising contacting the carbon monoxidewith an activated carbon supported catalyst produced by an improvedmethod as recited in claim 1, except that the activated carbon supportis impregnated with the inert hydrophobic compound used therein afterthe step of impregnation with the catalyst composition describedtherein.
 4. The method of claim 3 wherein in the method of producing theactivated carbon support catalyst the hydrophobic compound is one ormore members selected from the group consisting of aliphatichydrocarbons of at least fourteen carbon atoms and aromatic hydrocarbonsof at least ten carbon atoms.